1. Field of the Invention
This invention relates generally to light emitting devices, and more particularly to a lamp which produces white light from a light emitting diode and a phosphor composition.
2. Description of the Related Art
In recent years, significant improvements have been made in the fabrication of light emitting semiconductor devices which emit radiation in the shorter wavelengths. For example, Nichia Chemical Industries Ltd. in Japan has produced light emitting diodes and laser diodes which emit light in the blue and ultraviolet (UV) wavelengths. Nichia has also produced a light source which includes a blue light emitting diode (LED) and a phosphor. The phosphor, yttrium-aluminum-garnet doped with trivalent cerium, converts a portion of the blue light into a broad yellow emission. The blue and yellow light together produce white light having a color temperature of about 6000-8000xc2x0 K and a color rendering index (CRI) of about 77.
The Nichia light source, however, has a device luminous efficacy of only about 5 lumens per watt of input electric power. Thus, the luminous flux output by the Nichia lamp is relatively small, particularly if the input electric power is limited, e.g. by battery power. It would be desirable, therefore, to have a light source with a higher device luminous efficacy and luminous flux output for improved performance.
A lamp, according to an exemplary embodiment of the invention, comprises a light emitting element such as a light emitting diode or a laser diode which emits blue light, and a phosphor composition which absorbs the blue light having a first spectrum from the light emitting element and emits light having a second spectrum. The phosphor composition comprises at least one of Ba2MgSi2O7:Eu2+Ba2SiO4:Eu2+; and (Sr,Ca,Ba)(Al,Ga)2S4:Eu2+.
The invention also relates to a light emitting element which emits blue light, and a phosphor composition which absorbs the blue light from the light emitting element having a first spectrum and emits light having a second spectrum, wherein the spectral luminous efficacy of light emitted by the lamp is at least 550 lumens per watt of radiant power. The high spectral luminous efficacy of the output spectrum efficiently converts the power input to the lamp into lumens to provide a high brightness. For example the device luminous efficacy of the lamp can be 35-45 lumens per watt of input electric power or more.
The lamp is particularly useful as a flashlight where brightness may be an important characteristic of the lamp, input power may be limited, and color rendering is less important.